1. Field of the Invention
This invention relates to liquid crystal light valves (LCLVs), and more particularly to light valves designed to have a high resolution and dynamic range.
2. Description of the Related Art
Light valves, generally employing liquid crystals as an electro-optic medium, are used to spatially modulate a readout beam in accordance with an applied input signal pattern. They can be used to greatly amplify the input pattern by controlling a readout beam of much greater intensity, to convert spatially modulated incoherent radiation to a coherent readout laser beam with a similar spatial modulation, for optical data processing, wave-length conversion, or for other purposes that involve the conversion of an input signal pattern to a corresponding spatial modulation on a separate readout beam.
An earlier type of LCLV used a cadmium sulfide (CdS) photoconductor medium. This type of device is described, for example, in Grinberg et al., "A New Real-Time Non-Coherent to Coherent Light Image Invertor--The Hybrid Field Effect Light Crystal Light Valve", Optical Engineering 14, 217 (1975).
The main drawback of the CdS-based light valve has been its slow response time. A second generation silicon-based LCLV has been developed which retains the advantages of the CdS-based light valve and has a considerably faster response time. The silicon-based device is described in an article by Efron et al., "The Silicon-Liquid Crystal Light Valve", Journal of Applied Physics 57(4), pages 1356-68 (1985). This article also summarizes some of the prior light valve efforts.
The internal construction of a silicon-based LCLV is shown in FIG. 1. An input image beam on the input side of the device is identified by reference numeral 2, while a readout beam 4 is directed onto, and reflected from, the opposite side of the device. A layer of high resistivity silicon photoconductor 6 has a thin P.sup.++ back contact layer 8 formed on its input side. This back contact provides a high sheet conductivity to present a very small load at any point in the device's cross-section where carriers are generated. It also gives the device a linear operation, avoiding a situation in which the sensitivity and resolution are dependent upon the input light level, and provides higher output uniformity under dark conditions. An SiO.sub.2 oxide layer 10 is provided on the input side of back contact 8, with a fiber optic plate 12 adhered to the oxide layer by means of an optical cement 14. A DC-biased n-type diode guard ring 16 is implanted at the opposite edge of the silicon photoconductor wafer 6 from back contact 8 to prevent peripheral minority carrier injection into the active region of the device. An SiO.sub.2 gate insulator layer 18 is formed on the readout side of the silicon photoconductor 6. Isolated potential wells are created at the Si/SiO.sub.2 interface by means of an n-type microdiode array 20. This prevents the lateral spread of signal electrons residing at the interface, and thereby enhances the device's resolution.
A unified thin film dielectric mirror 22 is located on the readout side of the gate oxide layer 18 to provide broad-band reflectivity, as well as optical isolation to block the high intensity readout beam from activating the silicon photoconductor layer 6. A thin film of fast response liquid crystal 24 is employed as a light modulating medium on the readout side of mirror 22. A front glass plate 26 is coated with an indium tin oxide (ITO) counter electrode 28 adjacent the liquid crystal. The front of glass plate 26 is coated with an anti-reflection coating 30, and the whole structure is assembled within an airtight anodized aluminum holder. Silicon photoconductor 6 is shown coupled with oxide layer 18 and transparent metallic electrode coating 28 to form an MOS structure. The combination of the insulating liquid crystal, oxide and mirror act as the insulating gate of the MOS structure.
In operation, an alternating voltage source 32 is connected to one side of back contact 8 by means of an aluminum back contact pad 34, and on its opposite side to counter electrode 28. The voltage across the two electrodes causes the MOS structure to operate in alternate depletion (active) and accumulation (inactive) phases. In the depletion phase, the high resistivity silicon photoconductor layer 6 is depleted and electron-hole pairs generated by input light beam 2 are swept by the electric field in the photoconductor, thereby producing a signal current that activates the liquid crystal. The electric field existing in the depletion region acts to sweep the signal charges from the input side to the readout side, and thus preserves the spatial resolution of the input image. The polarized readout beam 4 enters the readout side of the light valve through glass layer 26, passes through the liquid crystal layer 24, and is reflected by dielectric mirror 22 back through the liquid crystal.
Since the conductivity of each pixel in photoconductor layer 6 varies with the intensity of input beam 2 at that pixel, a voltage divider effect results which varies the voltage across the corresponding pixel of the liquid crystal in accordance with the spatial intensity of the input light. As is well known, the liquid crystals at any location will orient themselves in accordance with the impressed voltage, and the liquid crystal orientation relative to the readout light polarization at any particular location will determine the amount of readout light that will be reflected back off the light valve at that location. Thus, the spatial intensity pattern of the input light is transferred to a spatial liquid crystal orientation pattern in the liquid crystal layer 24, which in turn controls the spatial reflectivity of the light valve to the readout beam.
An important function of the dielectric mirror 22 is to block readout light and prevent it from activating the photoconductor substrate 6. The intensity of the readout beam may be on the order of 10.sup.6 -10.sup.8 times the input beam intensity. During the active (depletion) phase of light valve operation, minority carriers are transported from the back face of the photoconductor layer to the readout face adjacent the dielectric mirror. It is this accumulation of a small quantity of spatially resolved carriers at the readout face that produces a voltage pattern for activating the liquid crystal layer. Since the photoconductor layer 6 is photosensitive, a dielectric mirror/light blocking layer 22 is required to prevent the high intensity readout light from generating spatially unresolved carriers in the photoconductor that would otherwise swamp the signal charge. Typically, the dielectric mirror/light blocking layer 22 must attenuate the readout beam by a factor of about 10.sup.6 or more, so that the number of carriers accumulated during the active phase due to light leakage through the dielectric mirror/light blocking layer does not approach or exceed the signal charge. It is quite difficult to fabricate a dielectric mirror with this capability. Although an attenuation of 10.sup.7 has been achieved, some applications require greater attenuations, for which adequate dielectric mirrors are not presently available.
As a possible substitute for a dielectric mirror, a recently developed metal matrix mirror has been demonstrated to provide good electrical and optical properties for LCLVs operating in the infrared region. This type of mirror is described in co-pending U.S. patent application Ser. No. 759,004, "Reflective Matrix Mirror Visible to Infrared Converter Light Valve", by P. O. Braatz, and assigned to Hughes Aircraft Company, the assignee of the present invention.
A metal matrix mirror is illustrated in FIG. 2. A matrix of reflected islands 36 is formed on an insulative layer 38, such as SiO.sub.2, on the silicon photoconductor substrate 40. The islands 36 are separated from each other so as to avoid short-circuits across the face of the mirror. The dimensions of the individual islands 36 are determined from a minimum size for adequate reflection, on the order of 5-20 microns, and the resolution or pixel element size for which the light valve is designed. The thickness of the islands depends upon the specific reflective material employed. There is a basic requirement that the free electron density of the reflective material be sufficient to interact with the readout radiation and scatter it back out of the material. Metals such as aluminum or silver or metal/semiconductor compounds such as platinum-silicide may be used.
Although the dielectric mirror version of the LCLV described above can be used over a fairly wide range of wavelengths, it suffers from a charge spillover phenomenon that exists in the microdiodes as a result of a lateral punch-through mechanism under higher illumination levels. This spillover, from an activated pixel to an adjacent non-activated pixel, reduces the dynamic range of the device and can degrade its resolution.
The metal matrix mirror version, on the other hand, has been limited principally to infrared radiation because of the bandgap of the silicon substrate. In the visible region, readout light leaks through the vacant channels separating the metal islands, causing activation of the underlying photoconductor. Since only about 70% of the readout surface is occupied by the reflective islands, enough light leaks through between the islands to effectively prevent operation in the visible region.
Another type of light modulator is disclosed in U.S. Pat. No. 4,619,501 to Armitage. In this device a solid electro-optic crystal is used instead of liquid crystals. Lithium niobate, lithium tantalate and potassium dideuterium phosphate are suggested for the solid crystal materials. A lattice of microgrooves is formed on the surface of the silicon wafer to prevent lateral charge movement. A light blocking layer is provided on top of the silicon wafer, followed by a dielectric mirror, and then the electro-optic crystal. Although seeking to prevent lateral charge movement between pixels, the open microgrooves can potentially become contaminated with moisture. This is turn can cause short circuits between adjacent pixels, a variable dielectric constant as the moisture content varies over time, and unstable light valve operation.